add forgotten files

lib/fun_core/basic_signup_allowlist.ex

@@ -0,0 +1,19 @@
+defmodule FunCore.BasicSignupAllowlist do
+  def normalize(email) do
+    email |> String.trim() |> String.downcase()
+  end
+
+  def addresses_as_list(addresses_str) do
+    addresses_str
+    |> String.split(",")
+    |> Enum.map(&normalize/1)
+  end
+
+  def mail_allowlisted_fun(signups_allowed, email_received) do
+    case signups_allowed do
+      nil -> false
+      "*" -> true
+      list_str -> normalize(email_received) in addresses_as_list(list_str)
+    end
+  end
+end